MINOR: connection: remove some unneeded checks for CO_FL_SOCK_WR_SH

MINOR: connection: remove some unneeded checks for CO_FL_SOCK_WR_SH

A few places in health checks and stream-int on the send path were still
checking for this flag. Now we do not and instead we rely on snd_buf()
to report the error if any.

It's worth noting that all 3 real muxes still use CO_FL_SOCK_WR_SH and
CO_FL_ERROR interchangeably at various places to decide to abort and/or
free their data. This should be clarified and fixed so that only
CO_FL_ERROR is used, and this will render the error paths simpler and
more accurate.